Enamel forms extracellularly on mesenchymally derived dentin matrix through the ordered assembly of a protein scaffold that regulates crystallite dimensions. Two studied proteins of the enamel matrix are amelogenin and tuftelin. We show amelogenin self-assembly depends on the amino-terminal 42 residues interacting with a 17 residue domain in the carboxyl region. We predict that animals expressing amelogenin proteins bearing deletions of either of these two assembly domains will have an adverse effect on enamel matrix assembly and hence upon mineral phase development. We predict that prism to prism defects in bulk enamel as well as dentino-enamel junction (DEJ) defects will be observed in transgenic mice mis-expressing amelogenins truncated to their assembly domains, but not in their non-transgenic litter mate controls. We predict that the enamel defects will be identifiable using novel testing methods. Ultimately, these transgenic animals will be useful models to study inherited enamel defects that affect humans such as amelogenesis imperfecta. Specific aims are: 1) To measure and compare enamel fracture toughness and hardness between normal and transgenic mice, 2) To measure and compare DEJ interfacial fracture toughness between normal and transgenic mice, 3) To identify, localize and compare DEJ failure mechanisms in normal and transgenic mice. These investigations will related a specific genetic defect in the amelogenin assembly domain of mis-expressing transgenic mice to resultant changes in their enamel structure. Structural changes in enamel will be quantified by measurement of fracture toughness and hardness in two perpendicular planes. This will provide important information about the spatial orientation of defects with respect to enamel prism orientation. Structural changes in enamel that compromise its ability to interact with dentin and form a normal DEJ will be quantified by measuring the interfacial fracture toughness of the DEJ. Specific failure mechanisms of the DEJ will be identified and related to enamel defects. Enamel defects related to the DEJ will be localized by fracture toughness and hardness profiling across the DEJ and adjacent tooth structures. In all cases, tooth structure from the mis-expressing transgenic mice will be compared to normal controls. Unlike prior studies of enamel defects on a few isolated individuals of unknown genetic etiology, or on defects produced by generalized poisoning, these studies are repeatable and the single genetic cause is known.